Terrestrial digital broadcasting service has been begun, and receivers for receiving the broadcasting service have been developed actively. Japanese Patent Unexamined Publication No. H11-112460 discloses an orthogonal-frequency-division-multiplex (OFDM) signal demodulator that can receive such broadcasting service and shorten the locking time of the frequency synchronization.
A conventional receiver will be described hereinafter with reference to the drawing. FIG. 8 is a circuit block of the conventional receiver. In FIG. 8, the receiver has the following elements:                input terminal 1004;        receiving unit 1001 connected to input terminal 1004;        demodulating unit 1002 connected to the output side of receiving unit 1001; and        output terminal 1040 connected to the output side of demodulating unit 1002.Receiving unit 1001 has the following elements:        phase locked loop (PLL) 1010;        mixer 1006 for mixing local oscillation signals from PLL 1010 and signals from input terminal 1004; and        filter 1008 connected to the output side of mixer 1006.PLL 1010 has the following elements:        local oscillator 1007 for supplying the local oscillation signals to mixer 1006;        variable frequency divider 1102 for frequency-dividing the local oscillation signals from local oscillator 1007;        crystal oscillator 1012; and        phase comparator 1104 that is connected to the input side of local oscillator 1007 and compares signals from variable frequency divider 1102 with signals from crystal oscillator 1012.The output part of phase comparator 1104 is connected to the input part of local oscillator 1007 via loop filter 1003.        
In the conventional receiver having such a configuration, variation in temperature causes a frequency error in signals of quartz crystal 1011. The frequency error in the signals of quartz crystal 1011 is about 100 ppm or lower. When the reference frequency of the signals of quartz crystal 1011 is assumed to be 18 MHz, for example, the frequency error range is ±1.8 kHz, extremely small. The output frequency of the local oscillation signals from local oscillator 1007 that are generated based on the error-included signals from quartz crystal 1011 includes a frequency error of the value derived by multiplying the error by the ratio between the output frequency of the local oscillation signals and the reference frequency of the signals of quartz crystal 1011. A large frequency error occurs in the output frequency of the local oscillation signals.
When the local oscillation signals including such a large frequency error that have been supplied from local oscillator 1007 are fed into mixer 1006, the selected intermediate frequency (IF) signals supplied from mixer 1006 have a similar frequency error. Therefore, filter 1008 for removing undesired signals from the IF signals supplied from mixer 1006 must have a pass band width of desired signals that is larger by a band width corresponding to the frequency error. As a result, the attenuation characteristic of filter 1008 is not steep, and the attenuation amount of unnecessary undesired signals existing near the frequency of the desired signals decreases inevitably.